The present invention pertains to the field of passive electronic components. In particular, the invention relates to thin film capacitors and methods for forming the thin film capacitors.
Integrated circuits (xe2x80x9cICsxe2x80x9d) are typically mounted on printed circuit boards that contain and interconnect other active (e.g., ICs) and passive components (e.g., capacitors, inductors, and resistors). Many of the prior art passive components are discrete passive componentsxe2x80x94i.e., individual capacitors, inductors, and resistors. Many prior art printed circuit boards use wire traces to interconnect ICs mounted on these boards and discrete passive components mounted on these boards.
There are a number of disadvantages, however, to using discrete passive components. Discrete passive components typically take up relatively large amounts of valuable space on printed circuit boards, which works against miniaturization. Discrete passive components also add cost. There is the cost of the discrete passive components themselves. There is also the cost of attaching multiple discrete passive components to a printed circuit board. A further disadvantage is that relatively long interconnection paths typically associated with discrete passive components can sometimes reduce switching speeds, especially as average switching speeds have increased.
Passive components (i.e., capacitors, inductors, and resistors) have, however, in the prior art been integrated on a single substrate with or without other ICs.
Anodization has been used in the prior art to produce integrated capacitors in an attempt to produce capacitors with relatively high capacitance density, relatively high breakdown voltage, relatively tight tolerances, and relatively low fabrication costs. The thin film integrated capacitors have been made by anodizing a continuous layer of metal and then selectively etching the anodized metal.
Because anodization is a self-limiting process, it is typically possible to obtain tight tolerances without the need for any post-process trimming. By picking certain metals to anodizexe2x80x94such as tantalum and aluminumxe2x80x94it is also possible to get relatively high breakdown voltages and relatively high capacitance densities. Anodization has been used to create aluminum decoupling capacitors for multichip modules. These aluminum coupling capacitors typically have relatively high capacitance density, relatively high breakdown voltages, and relatively tight tolerances. Tantalum oxide capacitors that have been produced typically have higher capacitance densities than aluminum capacitors but lower breakdown voltages.
Both aluminum and tantalum capacitors have been fabricated by anodizing a continuous layer of an anodizable metal and then patterning the metal by selectively etching away the area between the capacitors, a process known as subtractive etching. Although subtractive etching can successfully pattern capacitors, the process is often tedious and typically uses hazardous chemicals that are subject to strict environmental regulation. In addition, many of these processes are subject to problems with photoresist lifting. Successful manufacturing can require 100% inspection of the parts during manufacturing combined with a substantial yield loss from the parts that failed. For example, one process for preparing aluminum oxide capacitors uses an etchant containing CrO3, which is a heavily regulated chemical with possible carcinogenic properties. An alternative process for etching aluminum oxide uses hydrofluoric acid, which requires an intermediate hard bake during the etching process to avoid photoresist lifting. Many of the oxides that are produced during anodization are very hard to etch and typically require the use of hazardous or highly regulated chemicals.
Selective anodization has been used for various prior art processes. A disadvantage of prior art selective anodization has been that with photoresist as an anodization mask, the adhesion between the photoresist and the surface of the metal can be so weak during anodization that sometimes serious lateral anodization results. In other words, undercutting can be a problem. Thus, the anodized areas sometimes cannot be formed in an exact shape.
Different prior art approaches have been used in an attempt to improve the dimensional control of the anodized areas. For example, a barrier layer has been used to help prevent lateral oxide growth. Other examples involve the use of deposited insulators or anodizable metal as anodization masks. These approaches typically introduce more process complexity or increase reliability problems. For example, the deposition of an insulator can sometimes physically damage the top layer of anodic oxide, which can sometimes degrade the junction properties.
A thin film capacitor and methods for forming the same are described. The capacitor has a dielectric layer with a first face, a second face, and at least one edge. The first terminal of the capacitor covers at least a portion of the first face of the dielectric layer, covers at least a portion of one edge of the dielectric layer, and covers a portion of the second face of the dielectric layer. The second terminal of the capacitor covers a portion of the second face of the dielectric layer and does not contact the first terminal. The method for forming the thin film capacitor includes hard baking a photoresist at an elevated temperature and anodizing an exposed metal area using the photoresist as a mask.
Other features and advantages of the present invention will be apparent from the accompanying drawings and from the detailed description that follows below.